In conventional telephone systems, the telephones are powered from a central office of the particular telephone company, with a voltage being supplied to each telephone over a pair of telephone wires. The telephone lines in this type of service are often referred to as "plain old telephone service" (hereinafter POTS) quality lines. The central office normally supplies -48 V in an idle condition, typically referred to as "talk battery," when a ringing signal is not present. When an incoming call is initiated, the central office superimposes an AC ringing signal upon the normal DC voltage to generate a signaling or ringing noise on the customer telephone.
In these conventional telephone systems, it typically has been required to provide power to the customer telephones from the central office rather than locally, so that in the event of a local electrical power failure, the customer telephones remain operational. For a variety of reasons, however, it is desirable to provide power to customer telephones and generate ringing signals at a remote terminal located closer to customer premises. For example, the ability to power digital multiplexing equipment at a remote terminal close to the customer enables the telephone company to provide service to several customer devices. However, due to the high loop impedances associated with POTS lines, a central office would have to provide a high level of voltage in order to power remote digital equipment, supply talk battery, and power a remote terminal ringing generator.
Systems have been proposed for digitally multiplexing existing telephone systems having POTS quality lines in order to provide two complete telephone signals over a single pair of POTS lines, such as the development of 2B1Q technology. These digitally multiplexed telephone systems increase the utility of existing POTS lines, which previously served only a single customer device, to serve multiple customer devices having distinct telephone numbers using the existing single pair of POTS wires. However, in digitally multiplexed telephone systems, it is necessary to provide a regulated low voltage supply to the digital components located in a remote terminal. Thus, the use of digital multiplexing for telecommunications systems further necessitates the development of a highly reliable remote terminal power converter.
In digital multiplexing systems utilizing remote terminals, the central office must supply a voltage output capable of powering a remote terminal ringing generator, in addition to providing a talk battery supply voltage. For example, depending on changes in power demands at the remote terminal, and when considering the high loop impedance of POTS quality lines, the central office must provide voltage levels from about -78 V to as high 260 V (+130 V to -130 V). Thus, a successful and reliable remote terminal power converter must be capable of receiving and converting widely variable DC input voltages from the central office in order to provide a regulated output voltage to the multiple customer devices, a function which significantly complicates the design of remote power converters in telephone systems. Additionally, a successful remote terminal power converter must be capable of delivering a regulated low voltage supply (typically +5 V) to the logic circuits of the digital multiplexing components from the widely variable DC input voltages provided by the central office.
In order to provide the required output voltage to the particular loads, it is necessary to use some type of rectifier in the remote terminal power converter. Typically, a diode would be utilized. However, conventional diodes have a voltage drop when they conduct current that dissipates heat, depending on the RMS value of the current pulses through the diodes. The amount that the voltage drop of the rectifier diodes detracts from the overall efficiency of the converter is dependent mainly upon the level of the output voltage. For example, if the output voltage is relatively high, such as 48 V, then the diode voltage drop is insignificant. However, if the required output voltage is low, such as +5 V for digital logic circuits, then the diode voltage drop becomes a significant factor in the inefficiency of the converter.
In power rectifier applications, conventional silicon diodes have a voltage drop of about 0.8 V, while Schottky diodes have a voltage drop of about 0.6 V. Additionally, conventional diodes have a relatively high reverse voltage breakdown, while Schottky diodes have a relatively limited reverse breakdown voltage, a maximum of 60 V. As is apparent, both of these types of diodes would significantly decrease the efficiency of a DC--DC converter which is used to deliver a regulated low voltage output. Thus, it is desirable to replace the conventional diode with some other type of rectifier in order to maximize the converter efficiency by minimizing the rectifier voltage drop. One possibility is the use of a synchronous rectifier, which could utilize any number of power switching elements to perform the rectifier function, such as a power transistor or a power field effect transistor (FET).
Synchronous rectifiers are typically used with transformers which couple large amounts of power to output loads. In this environment, a significant advantage with the use of synchronous rectifiers is that they produce far less heat dissipation associated with high power coupling. In order to operate properly, however, the synchronous rectifier must operate in a continuous mode. In other words, energy must always be either flowing into or out of the core of the transformer.
In a situation as described above in which it is desired to provide a low voltage regulated supply from a widely variable DC input voltage, the use of a synchronous rectifier in a remote terminal power convertor becomes greatly complicated. Because of the wide variations in the input voltages from the central office and the output load demands, a discontinuous interval arises. This discontinuous interval occurs when energy is neither flowing into or out of the core of the transformer, thus allowing the transformer to oscillate at its self-resonant frequency. The discontinuous interval has a variable time duration dependent on the input voltage level being coupled to the remote terminal power converter from the central office, and the power demanded from the output loads. Thus, in order to provide a suitable low voltage output to the low voltage loads, it is necessary to prevent the discontinuous interval from affecting the output voltage regardless of the duration of the discontinuous interval, a function which significantly complicates the design and use of a synchronous rectifier in a power converter.